FIG. 1 illustrates a prior art tool for piercing nuts which is adapted to fasten piercing nuts to metal sheet. This requires great accuracy for the nut or metal sheet so as to not damage the metal sheet and for the nut to be safely permanently connected to the metal sheet. A condition for such accuracy is that the punch of the tool for piercing nuts bottoms with the correct pressure against the piercing nut, i.e., that the stroke of the punch is correctly set for the used piercing nut and the metal sheet in question. To achieve this accuracy within some tenths of a millimeter, spacers, or shims, are currently used for adjustment in the shown prior art tool for piercing nuts. The shims are arranged between an adjusting plate and the tool for piercing nuts which thus is moved in its entirety by means of the shims. This mode of operation is time consuming and still does not provide exact information about the pressure to which the piercing nut and the metal sheet are subjected. A negative effect on, for instance, the strength of the piercing nut owing to a punch pressure which after all is incorrect, thus cannot be excluded. Moreover, it is necessary to completely stop the use of the tool for piercing nuts during the actual adjustment, which causes expensive production standstill when the tool is included as one of a plurality of tools in a long press line.